1. Field of the Invention
The present invention relates to a process and apparatus for the production of panels, webs, panellike moldings or profiles from plastic materials, preferably thermoplastics, of high mechanical strength. The process provides a molecular orientation by compressive roll-stretching within the nip of at least one pair of pressure or compression rolls at roll and material temperatures below the crystallite melting point or the softening point of the plastic and above 303.degree. K. under compressive stretching. With the aid of this process and the apparatus, the mechanical properties of the panels, webs, panel-like moldings or profiles can be successfully improved.
2. Description of Related Art
European Patent No. 0 051 730 discloses a process for the production of panels, sheets, webs, strips, profiles or panel-like or sheet-like moldings of high mechanical strength from thermoplastic materials by compressive stretching within the nip of several pairs of pressure rolls under the crystallite melting point or softening point of the thermoplastic. At least one pair of compression rolls of the compression roll unit used for the compressive stretching briefly exert a line load of 100 N/cm-900 N/cm, preferably 150 N/cm-850 N/cm, on prefabricated panels, sheets, webs, strips, profiles, panel-like or sheet-like moldings or on panel-like, sheet-like, web-like, strip-like or profile-like preforms, coming out of the extruder, in the nip of the pair of compression rolls. After the first brief compressive stretching operation, while still within the regeneration time of the fatigue effect of the pressurized region and/or the relaxation time of the elastic recovery of the pressurized region of the thermoplastic, following the expiry of a time interval of between 5 s and 5 min, preferably between 20 s and 3 min, the workpieces are subjected to one or more further brief compressive stretching and/or subsequent compressive handling operations in the nip of one or more further pairs of compression rolls with a smaller line load of 75 N/cm to 700 N/cm, preferably 80 N/cm to 650 N/cm. The temperature of these rolls or pairs of rolls is equal to or less than the temperature of the first compression roll unit used for compressive stretching, the pressure is varied as a function of the nip, and the nip preferably has the same width as the upstream pair of compression rolls for compressive stretching.
However, in the application of this process it has been found to be disadvantageous that, when using the line loads of 100 N/cm to 900 N/cm, the desired properties are not achieved to the full extent in the case of all thermoplastic materials and in the case of all roll temperatures specified within the temperature range and/or relatively frequent or numerous passes may become necessary within the relaxation period. Furthermore, there is the difficulty that, to achieve the specified advantages, depending on the thermoplastic and temperature, usually only panels up to a small width, for example, a strip up to 20 cm or up to 25 cm, could be compressively treated.